Add command line flags to init-repository.py

This patch adds --upstream (-u) and --snapshot (-s) flags to the
init-repository script which make it possible to choose the
Chromium sources to use when building QtWebEngine. The default
is to use the upstream submodules.
This option is persisted with git config and used throughout the
build system.
This patch also keeps support for the CHROMIUM_SRC_DIR env
variable which can be used to specify external Chromium
sources which are assumed to be set up for the build and
thus left untouched by the script.

Change-Id: Ic83984199770076f90867f1b22a63b220bfe7359
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>